The fate tests change as they used 1.2 previously
The increased size is due to:
32bit CRCs per slice by default (can be disabled),
it adds slice headers to allow decoding one slice without the others
an additional slice size field is added to make it possible to find
slices within corrupted surroundings.

these add up to about 57bit per slice more
at 50 frames and 4 slices thats 1425 byte